Yes, I think it has to be van Persie for that sickener in that 3-2 defeat. As for Long or Lukaku that has to be more difficult but I am going for Long as we were dreadful in that second game!
Van Persie and Long, pushed by Lukaku. Gervinho was on fire against us at The Emirates but we were crap.
RVP wins hands down for me. Long beats Lukaku on the basis that Long also caused us some problems in the away game, not just the home game. He chased down that long ball forward, before crossing it for Odemwingie to score WBA's second, as I recall.
Long is a terrier, but I'm opting for that extra quality in RVP & Lukaku Not mentioned yet but Suarez was great at Anfield, but was at the wrong end of a very good performance from ourselves at St Mary's
I did say we should loan him from Chelsea before West Brom stole my idea. I count it as some of my best work (not that it was terribly incisive and inventive). With reference to the OP - Van Persie and Lukaku.